If you get hives, you should still wear sunscreen under the arm coolers. If it's true that they only block 75% of UV, then that's only an SPF of 4. (if you burn after 15 min in the sun, these will block enough that you won't burn until after 1 hr)
Whatever your tolerance is before hives, you can multiply by 4.
Do check out the Sun Precautions site. Their Solumbra fabric is like an SPF of 50 or 55, and it's really very light. My vented shirt is very cool and comfortable, and my Pearl Izumi vest keeps it from flapping around in the wind.
This is my Kokatat shirt (though mine is an older model)
http://www.kokatat.com/product_detail.asp?code=wdsh
I ride in it, or in my long sleeved wool jerseys. With sunscreen underneath because I can burn in 5 or 10 minutes. I like it because I can really move my arms around, good for when I kayak.



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I'm a 3-pocket gal when it comes to my long rides... but otherwise it's perfect!
